GLACIAL LAKES 



329 



Episodes of the glacial lake history within the St. Lawrence 

 valley. Within this great drainage basin it has apparently 

 been possible to read the records of each stage in the latest lake 

 history complex as this has been. We have only to recall the 

 lake stages cited from the Scottish glens and remember that each 

 new stage was begun in a retirement of the glacier front which un- 

 blocked an outlet of lower level than the last. This sequence 

 might, however, have been varied by a temporary readvance of the 

 ice, as indeed once occurred in the Huron-Erie lobe of the great 

 North American glacier. 



The crescentic lakes of the earlier stages. So long as the 

 glacier covered the entire drainage basin of the St. Lawrence 



PIG. 358. The continental glacier of North America in an early stage of its reces- 

 sion, when it covered the entire St. Lawrence drainage basin. The dashed line 

 is the approximate position of the divide (based on a map by Goldthwait). 



River system, all water was freely drained away by streams which 

 flowed away from the ice front (Fig. 358). So soon, however, 

 as at any point the front had retired behind the divide, impound- 

 ing of the waters must locally have occurred. Lakes of this type 

 are to-day to be seen in Greenland and in the southern Andes ; 

 and though upon a diminutive scale, some idea of their aspect may 

 be obtained from the appearance of the Marjelen Lake of Swit- 

 zerland, here blocked by a mountain glacier (Fig. 446, p. 411). 



